1. Field of the Invention
The present invention relates to a method and system for backing up data from a plurality of disks, and particularly to interleaving the data from the disks during backup, thereby decreasing the time to perform a backup as well as a restore.
2. Related Art
Backing up data from one or more computer disks is typically performed to recover from inadvertent user deletions/overwrites or from disk hardware failure. In the case of inadvertent user deletions/overwrites, only the data corresponding to the destroyed data is copied to the original disk from the backup. In the case of disk hardware failure, the user can restore all files to the original disk from the most recent backup.
In most computer systems, the backup device is a tape drive, which can accommodate large amounts of data at a relatively low cost per byte of storage. However, because a tape drive is fundamentally a sequential access medium, random access or adjusting backward/forward takes significantly longer for the tape drive compared to the disk drive. Therefore, the most efficient way to use a tape drive is to “stream” the data, i.e. ensure that the tape drive does not have to stop until the backup or restore is complete.
Generally, conventional backup methods provide for either file-by-file backup or image backup. In a file-by-file backup, the backup program copies one file at a time from the disk to the tape. Specifically, the program places all pieces of data for each file, irrespective of actual locations on the disk, into a single sequential block that is stored on the tape. Thus, a file-by-file backup can provide an incremental backup (wherein only those files that have changed since the last backup are written to tape), but is extremely time consuming for a full backup.
In an image backup, the data image is read sequentially from the disk and written to the tape. Thus, in prior art systems in which the disk drive is substantially faster than the tape drive, an image backup can keep the tape drive streaming. However, current technology has significantly improved tape drive speed. In fact, in state of the art systems, the tape drive speed is actually equal to or greater than the disk drive speed. In these systems, the tape drive cannot stream and thus begins to degrade both backup and restore performance.
Therefore, a need arises for backup and restore operations that can take advantage of technology improvements in tape drive speed.